This research is based on compelling evidence that the variable sensitivity of human solid tumors to bifunctional antineoplastic alkylating agents correlates with the extent of formation or rate of removal of DNA interstrand crosslinks. Project goals are 1) to identify tumors in which DNA repair activity determines response to three alkylating agents - melphalan, cyclophosphamide and ifosfamide and 2) to elucidate the enzymic reactions and molecular mechanisms underlying such repair of DNA crosslink lesions. We shall exploit the broad range of responsiveness displayed by a series of pediatric rhabdomyosarcoma xenograft lines to the three alkylating drugs. Tumor lines identified by use of the alkaline elution technique as resistant due to elevated DNA repair activity will be studied in relation to their sensitive counterparts. Substrates for in vitro assay of DNA repair enzyme will be made by reacting DNA with melphalan or activated derivatives of cyclophosphamide and ifosfamide. Crosslinking will be determined in vitro by measuring the renaturability of DNA with filter-binding or fluorometric methods. Potential repair enzyme activities will be determined comparatively in resistant and sensitive tumor lines. The overall goal of the project is to correlate levels of repair enzymes, DNA crosslink levels and cytotoxicity. Such information should enable prediction of therapeutic response based on direct biochemical assay of repair enzyme levels in tumor and normal tissues, using antibody or gene probes. Ultimately, intervention in repair processes by enzyme inhibition could provide an approach to overcoming drug resistance in patients with solid tumors.